Sale

<?php

$period = 5;

function ex($period)
{
    $daysInMonth = date('t');

    $daysOfEx = [$daysInMonth];

    for($i = $daysInMonth - 1; $i > 0; $i --) {
        if($i % $period == 0 && $daysInMonth - $i >= $period) {
            $daysOfEx[] = $i;
        }
    }

    return $daysOfEx;
}

print_r(ex(5));